1. Field of the Invention
The present invention generally relates to a medical X-ray imaging. More particularly, the present invention relates to a mammography apparatus wherein the tube collimator is adjusted automatically to provide a collimated X-ray beam that is appropriate for the dimension and position of a compression paddle to reduce the potential for scattered X-rays that may interfere with image quality.
2. Description of the Prior Art
As is well understood by those skilled in the art, sharp contrast and high resolution images are highly desirable in virtually all X-ray imaging applications. In medical X-ray imaging, sharp contrast and high resolution images are necessary to enable as accurate medical diagnosis as possible. As compared with some other medical X-ray imaging, such as the imaging of skeletal structures, mammography demands very sharp contrast and high resolution images because a radiologist must be able to distinguish between tissues having relatively small differences in X-ray absorption rates.
Scattered X-ray radiation (sometimes referred to as secondary or off-axis radiation) is a problem in X-ray imaging. Scattered radiation causes a loss of image contrast. Scattered X-ray radiation is a particularly serious problem in the field of mammography where high image contrast is necessary for the detection of very subtle changes in breast tissue. Scattered or secondary radiation can be caused by many sources, even the object under radiographic examination. In mammography, scatter is typically reduced by scatter reducing grids which only permit primary or on-axis radiation to reach the image receiver. However, particularly in mammography, despite using scatter reducing grids, it is important to reduce the potential for scatter radiation by limiting the size of the X-ray beam. This is typically accomplished through collimation.
A mammography apparatus is typically used to take both magnified and unmagnified mammographic images, and full field and spot images. Even with the range of views typically used, the prior art devices only employ two preset beam collimations, one for full field and the other for spot images. Although spot imaging can be either magnified or unmagnified, the prior art devices do not collimate the beam differently for either spot imaging technique. Instead, an arbitrary spot collimation setting is used.
Whether a breast is undergoing magnified or unmagnified spot imaging, or full field of view imaging, the distance between the compression paddle and the breast support (either the image receiver or another breast supporting device) varies with the thickness of the breast under compression. Prior art devices do not collimate the beam differently for breasts of different thicknesses. Ordinarily, the collimation of the beam in the prior art apparatus is set for an arbitrary collimation setting which may be for an average breast thickness. However, if the apparatus is set up to collimate the X-ray beam for an average breast thickness, as done by the prior art mammography apparatus, the X-ray beam collimation will not be optimized for thin breasts nor thick breasts. At either extreme, image quality can be affected. For example, with a thin breast, the X-ray beam area at the compression paddle may be larger than the compression paddle area, which may be undesirable because it can cause additional X-ray scattering, reducing image contrast. With a thick breast, the X-ray beam area may be smaller than the compression paddle area which may result in less than optimal imaging area.